U.S. corn futures fell on Tuesday, retreating from a three-week high as investors took profits a day after a U.S. government report on corn planting eclipsed trade forecasts, raising the likelihood of a bumper crop this fall.

Soybeans traded mixed, with nearby contracts pressured by profit-taking and deliveries of the cash product against spot May futures. New-crop values garnered support from worries about declining South American crop production following a severe drought and persistent sales of U.S. soy to China.

Wheat slumped on spillover pressure from weaker corn and soybeans and as beneficial weather boosted U.S. crop prospects. U.S. corn planting advanced more than expected last week despite cool weather and rain as a huge seeding effort in top producer Iowa propelled sowing of the total crop past the halfway point, a U.S. Department of Agriculture report late on Monday showed.

"Corn planting was well ahead of the trade (forecast) and almost twice the average pace for May 1. If you've got 40 percent of the crop planted by May 1, your chances for above-trend yield tends to increase," said Karl Setzer, analyst with Max Yield Cooperative in West Bend, Iowa.

The USDA's crop progress and conditions report revealed that 53 percent of the U.S. corn crop was planted as of Sunday, topping the average forecast of 43 percent from a Reuters poll of 18 analysts and well ahead of the five-year average of 27 percent complete.

Chicago Board of Trade May corn fell 1/2 cent to $6.59-3/4 per bushel, May soybeans lost 5-1/4 at $14.97-3/4 and May wheat declined by 14 to $6.33-3/4.

"The market is shifting its attention from the old crop to the new crop. We're seeing some of the inverse coming out of the soybeans today," Setzer said.

Deferred contracts were supported by persistent demand from China, which bought 110,000 tonnes of new-crop U.S. soybeans via private deals, according to a USDA announcement on Tuesday. The USDA also confirmed 220,000 tonnes of new-crop sales to China on Monday.

China is likely to buy even more U.S.-origin soybeans in the near term because of poor South American crops, Hamburg-based oilseeds analysts Oil World said. Soybean plantings were more in line with analyst projections at 12 percent complete as of Sunday, versus 6 percent the prior week and the 5 percent five-year average. Analysts had expected 13 percent of the crop to be planted, just shy of the record pace of 15 percent set two years ago.

Wheat fell in sympathy with corn, snapping a three-session rising streak despite concerns about the outlook for crops in Russia, Ukraine and Australia. U.S. winter wheat crop ratings improved slightly last week as the USDA pegged 64 percent of the crop in the good-to-excellent category, up 1 point from the prior week.